Digital data, voice and video communication within closed user groups with a central structure (companies with subsidiaries, retail chains with many affiliates, mainframe systems with several user levels) can be advantageously operated using radio communication via satellites. The switching and organization functions in these networks are carried out by a central station (hub). The antenna size and transmission power of the hub are considerably greater than those of the other member stations. Since member stations' satellite dishes usually have a very small diameter (Very Small Aperture Terminal, VSAT), such a satellite network is usually referred to as a VSAT network. A description of a typical VSAT network is given in Supplement 3, "Handbook on Satellite Communication: VSAT Systems and Earth Stations," ITU-R, Geneva 1994, which is hereby incorporated by reference herein.
The data flow from the hub to the VSATs (outroute) is received by all the members in the network; the signals sent by the individual VSATs (inroute) are usually of a lower speed and can only be correctly received by the hub due to the limited size of the VSAT stations. Communication between VSAT stations usually takes place via the central station (hub), referred to as a double hop, which causes an additional up to half-second delay in the signal transmission time due to the great distance to the satellites.
A specific advantage of such VSAT networks includes the fact that messages destined for a plurality of members need to be sent only once and can be received by all destinations simultaneously. By installing the communication equipment in the immediate proximity of the user, complex routing over several switching stations can be avoided, which can be considerable especially for international terrestrial communications.
Due to the signal attenuation caused by atmospheric effects, typical for radio frequencies higher than 10 GHz, a certain power margin must be provided when operating such systems, so communications do not immediately break down when attenuation occurs. In order to guarantee 99.5% availability over a year, this margin must be approximately 2-3 dB for the currently common "Ku band" transmission frequencies of 12-14 GHz. This margin is permanently built in through the dimensioning of the satellite network and therefore cannot be used for other transmission purposes in today's systems during the greater part of the year, when atmospheric path attenuation is much lower than the margin provided.
By providing a satellite link within a satellite-supported network with an inflexible fixed attenuation margin, a certain degree of reliability (availability) is achieved at the expense of system economy.